Measured upwelling radiances from Nimbus-7 SBUV
(Solar Backscatter Ultraviolet) and seven NOAA SBUV/2
instruments have been used to calculate the 340 nm Lambertian
equivalent reflectivity (LER) of the Earth from 1979 to
2011 after applying a common calibration. The 340 nm LER is highly
correlated with cloud and aerosol cover because of the low surface
reflectivity of the land and oceans (typically 2 to 6 RU, reflectivity
units, where 1 RU = 0.01 = 1.0%) relative to the much higher
reflectivity of clouds plus nonabsorbing aerosols (typically 10 to 90 RU).
Because of the nearly constant seasonal and long-term 340 nm surface
reflectivity in areas without snow and ice, the 340 nm LER can be used to
estimate changes in cloud plus aerosol amount associated with seasonal and
interannual variability and decadal climate change. The annual motion of the
Intertropical Convergence Zone (ITCZ), episodic El Niño Southern Oscillation
(ENSO), and latitude-dependent seasonal cycles are apparent in the LER time
series. LER trend estimates from 5° zonal average and from
2° × 5° , latitude × longitude, time
series show that there has been a global net decrease in 340 nm cloud plus
aerosol reflectivity. The decrease in cos<sup>2</sup>(latitude) weighted average
LER from 60° S to 60° N is 0.79 ± 0.03 RU over
33 yr, corresponding to a 3.6 ± 0.2% decrease in LER. Applying a
3.6% cloud reflectivity perturbation to the shortwave energy balance
partitioning given by Trenberth et al. (2009) corresponds to an increase of
2.7 W m<sup>−2</sup> of solar energy reaching the Earth's surface and an increase
of 1.4% or 2.3 W m<sup>−2</sup> absorbed by the surface, which is partially
offset by increased longwave cooling to space. Most of the decreases in
LER occur over land, with the largest decreases occurring over the US
(−0.97 RU decade<sup>−1</sup>), Brazil (−0.9 RU decade<sup>−1</sup>), and central
Europe (−1.35 RU decade<sup>−1</sup>). There are reflectivity increases near
the west coast of Peru and Chile (0.8 ± 0.1 RU decade<sup>−1</sup>), over
parts of India, China, and Indochina, and almost no change over Australia.
The largest Pacific Ocean change is −2 ± 0.1 RU decade<sup>−1</sup> over
the central equatorial region associated with ENSO. There has been little
observed change in LER over central Greenland, but there has been a
significant decrease over a portion of the west coast of Greenland. Similar
significant decreases in LER are observed over a portion of the coast of
Antarctica for longitudes −160° to −60° and
80° to 150°.